Active matter in space

Abstract

In the last 20 years, active matter has been a highly dynamic field of research, bridging fundamental aspects of non-equilibrium thermodynamics with applications to biology, robotics, and nano-medicine. Active matter systems are composed of units that can harvest and harness energy and information from their environment to generate complex collective behaviours and forms of self-organisation. On Earth, gravity-driven phenomena (such as sedimentation and convection) often dominate or conceal the emergence of these dynamics, especially for soft active matter systems where typical interactions are of the order of the thermal energy. In this review, we explore the ongoing and future efforts to study active matter in space, where low-gravity and microgravity conditions can lift some of these limitations. We envision that these studies will help unify our understanding of active matter systems and, more generally, of far-from-equilibrium physics both on Earth and in space. Furthermore, they will also provide guidance on how to use, process and manufacture active materials for space exploration and colonisation.

Fig. 1. Examples of active matter systems.

Overview of active matter systems on all length scales, ranging from nanoscopic molecular and colloidal systems to swarms of animals or robots, and even to human crowds. Despite their seemingly large differences, all these systems exhibit striking similarities regarding their emergent behaviours. Being out of thermodynamic equilibrium, active particles can explore complex forms of dynamical self-organisation and exhibit intricate interplays between single-particle properties and collective behaviours that are impossible at thermodynamic equilibrium. Examples of biological active matter systems (from left to right): biomolecular motors (Reprinted by permission from Springer Nature Customer Service Centre GmbH: Nature, M. Schliwa and G. Woehlke, Copyright 2003 Springer Nature); microorganisms such as motile bacteria and spermatozoa (Reprinted figure with permission from C. Bechinger et al.. Copyright 2016 by the American Physical Society); bacterial rafts and biofilms (Reprinted from H. Jeckel et al.; use permitted by authors of original publication); insects such as termites (From J. Werfel et al.. Reprinted with permission from AAAS); herds of mammals and human crowds (Reprinted from T. Vicsek and A. Zafeiris, Copyright 2012, with permission from Elsevier). Examples of artificial active matter systems (from left to right): reconstituted active microtubule networks (Reprinted by permission from Springer Nature Customer Service Centre GmbH: Nature, T. Sanchez et al., Copyright 2012 Springer Nature); Janus particles; living crystals (From J. Palacci et al.. Reprinted with permission from AAAS); robotic builders (From J. Werfel et al.. Reprinted with permission from AAAS); swarming robots (Reprinted from Rubenstein et al., Copyright 2014, with permission from Elsevier).

Fig. 2. The effects of gravity on active matter.

On Earth, microscopic active matter is affected by gravity in multiple ways. The most relevant ones include: a Sedimentation and creaming, where the balance between the particle’s and solvent’s densities lead to either an upward or downward motion of the particle in a fluid. F_v and F_g represent the viscous force and gravitational force, respectively. b Convection, where flow patterns at speed u due to density mismatches of various origin transport matter within the sample. The colour scale represents a gradient of density ρ going from low density (ρ_low) at the bottom to high density (ρ_high) at the top. c Gravitaxis, where a motile organism or synthetic particle with self-propulsion speed v moves in a direction influenced by gravitational fields.

Fig. 3. Examples of optical microscopy setups for microgravity research.

a FLUMIAS is a miniaturised high-resolution 3D fluorescence microscope coupled to a centrifuge on the International Space Station (ISS) for live cell imaging. FLUMIAS can therefore be operated in microgravity as well as in controlled artificial gravity conditions with an effective gravitational acceleration of up to 1.1×g. Reprinted from C. S. Thiel et al.; use permitted by authors of original publication. b The RAMSES flight platform is designed for the sounding rocket MAPHEUS to study the motion of self-propelled particles in space. Reprinted from R. Keßler et al., with the permission of AIP Publishing. c Optical microscopy setup used on the Airbus Zero-G for the acoustic manipulation of dense gold nanorods samples in microgravity. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer, G. Dumy et al., Copyright 2020 Springer Nature.

Fig. 4. The future of space exploration for active matter.

In the near future (<3 years), we can expect an increase in active matter experiments on the International Space Station (ISS) in low Earth orbit (LEO) based on using or adapting existing instrumentation for soft matter studies in microgravity. The main advantage of running active matter experiments on the ISS is the possibility of studying these systems under microgravity conditions for up to several hours. In the mid (5–10 years) and long term (>10 years), fitting this instrumentation on Moon, Mars and Beyond-Low-Earth-Orbit (BLEO) missions (e.g., on the Lunar Gateway) will enable testing active matter for the future of space exploration and colonisation on even longer time scales (up to a few years or longer). Moon picture credit: ESA/Hubble; use permitted under a CC-BY 4.0 licence. Mars picture credit: ESA/Hubble; use permitted under a CC-BY 4.0 licence.